The present invention relates to a surface acoustic wave device.
Recently, surface acoustic wave devices have been used as band pass filters delay lines, or the like. Some examples of conventional surface acoustic wave devices will be described below with reference to FIGS. 1 through 3, of the accompanying drawings.
FIG. 1 illustrates a fundamental construction of a surface acoustic wave device serving as a band pass filter. This surface acoustic wave device comprises a substrate 1, which is made of a piezoelectric material, such as lithium niobate (LiNbO.sub.3) or lithium tantalate (LiTaO.sub.3), and is provided on the surface thereof with an input transducer 2 and an output transducer 3, each of which is composed of a pair of interdigitated comb-like electrodes made of a thin film of metal. When electrical signals from a generator 4 are applied to the electrodes of the input transducer 2, the latter emits surface acoustic waves 5, which are propagated to the output transducer 3 and then converted into other electrical signals which can be taken out from a load impedance 6. The electrodes of the input and output transducers can be designed in a variety of patterns, so as to provide various desired characteristics to the filter when electrical signals are converted into acoustic signals or vice versa. However, there is a problem in that the input transducer emits bulk waves, in addition to surface acoustic waves, which bulk waves are propagated through the body of the substrate to reach the output transducer, thereby adversely affecting the output characteristics of the latter.
In order to eliminate the above mentioned problem, in another conventional device illustrated in FIG. 2, the input transducer 2 and the output transducer 3 are arranged diagonally opposite each other, and a multistrip coupler 7, which includes a plurality of parallel conductors, is provided between the input and output transducers. According to this arrangement, the path of propagation of the surface acoustic waves is changed as indicated by a solid arrow, and as a result, the effect of the bulk waves which are propagated as indicated by a dotted arrow, is eliminated.
In the conventional surface acoustic devices as mentioned above, however, there is a further problem in that a number of reflections of the surface acoustic wave occur and deteriorate the filter characteristics. It has been found from the results of some experiments that the reflections of the surface acoustic wave include reflections from portions of acoustical discontinuity on the substrate surface, such as, for example, reflections 10 and 10' from the ends of the multistrip coupler 7 and the output transducer 3 as illustrated in FIG. 3.